Interpreting and Modelling Interferograms
Tim Wright
COMET,
School of Earth and Environment,
University of Leeds
t.j.wright@leeds.ac.uk
Juliet Biggs,
COMET,
Department of Earth Sciences,
University of Oxford
juliet.biggs@earth.ox.ac.uk
Aims
 To improve understanding of how to interpret interferograms.
 To understand the displacement fields produced by different types of earthquakes and
magmatic systems.
 To learn how to produce elastic models for simple sources.
 To gain an appreciation of the uncertainties involved with determining source
mechanisms from InSAR data.
Introduction
Since the launch of ERS-1 in 1991, deformation from many types of tectonic and magmatic
events has been captured by InSAR. This includes earthquakes, volcanic inflation and
deflation, dike intrusions, landslides and hydrothermal systems. Earthquakes can be measured
ranging from Magnitude ~5 to Magnitude ~8 in size, and have a variety of different
mechanisms (e.g. thrust, strike-slip, normal). A wide variety of different source shapes have
been proposed for magmatic deformation, including the point source model (Mogi), pennyshaped crack, spheres and spheroids (oblate or prolate), and dykes. Typically, interferograms
are interpreted by comparing the interferograms with the predictions of analytical models, in
which displacement occurs on a simple source geometry (e.g. dislocation, sphere) in an elastic
half space.
In this practical class, you will examine 4 interferograms from different tectonic settings. For
each one, you will try to find a simple, single-element model that provides a reasonable fit to
the observed interferograms, using some simple elastic modelling codes based on the
analytical expressions of Okada (1985) and Mogi (1958).
Further reading
Dzurisin, D., 2007, Volcano Deformation: Geodetic monitoring Techniqes, Praxis Publishing
Ltd, pp 441.
Y. Okada, 1985. Surface deformation due to shear and tensile faults in a half-space. Bull.
Seism. Soc. Am., 75, 1135-1154
Mogi, K., 1958. Relations between eruptions of various volcanoes and the deformation of the
ground surfaces around them, Bull. Earthquake Res. Inst., 36, 99–134
Wright, T.J., 2002. Remote monitoring of the earthquake cycle using satellite radar
interferometry. Phil. Trans. R. Soc. Lond. A, 360, 2873-2888.
Other InSAR studies can be downloaded as pdf files from
http://www.see.leeds.ac.uk/~eartjw/papers/
Procedure
For each event,
1. Examine the handout. It contains 4 images:
(top left) Wrapped interferogram. Each interference fringe is a phase change of 2π
radians, which corresponds to motion of 28.3 mm in the satellite line of sight (the line of sight
is defined by the heading and incidence angle in this case). A change in colour from red
through yellow to blue, indicates motion away from the satellite, which can be interpreted as
subsidence only if the displacements are vertical.
(top right) Unwrapped interferogram. This is included to help interpret the interferograms.
Blue colours indicate motion away from the satellite (range increases); red colours indicate
motion towards the satellite (range decreases).
(bottom left) Zoom in on the wrapped interferogram, to help you count fringes.
(bottom right) Coloured and shaded topographic map for the area of the zoomed
interferogram.
2. Draw on a vector corresponding to the approximate horizontal projection of the vector from
the satellite to the ground. First draw a long arrow for the satellite then add an arrow showing
the look direction (remember, the current satellites always look to the right).
3. Count the fringes. How much motion is there towards and away from the satellite? Is it
symmetrical (i.e. is the motion away from the satellite equal to that towards the satellite, or
different). Note that dip-slip earthquakes have a very asymmetrical displacement pattern: i.e.
the subsidence will be larger than the uplift for normal-faulting earthquakes.
4. Determine approximately the type of event. Draw on the surface rupture (if there is one). If
appropriate, determine the direction of dip.
5. Try to produce a model for the earthquake (I suggest working in pairs).
i) Enter matlab and open the file trieste_defo.m by typing ‘edit trieste_defo’ . The file
begins with some comments on the purpose of the code, followed by a section of parameters
for you to alter, some fixed parameters and finally the matlab code itself.
ii) Use the information you worked out from looking at the interferogram to select a
source type and enter some approximate values for the appropriate parameters. Set the
incidence and heading angles appropriate for this interferogram.
iii) Run the code by typing ‘trieste_defo’. Both wrapped and unwapped simulations
will appear - compare these to the interferogram.
iv) Look at the 3D displacements (bottom left figure) and try to understand why the
line-of-sight deformation looks like it does.
v) Adjust the parameters to try to make the model a closer fit to the interferogram and
repeat steps iii to v until you have a satisfactory model.
Additional questions you might like to think about
1. Can you get a good fit to the data? If not, why not?
2. Is there more than one source type that can produce a fit to the data? If so, is it possible to
distinguish between them using this interferogram? Is there any other data that would help?
3. How sensitive are the interferograms to different parameters (e.g. experiment with different
values for fault rake).
4. How do these events relate to geomorphic features in the landscape?